Exposure apparatuses are commonly used to transfer an image from a reticle onto a semiconductor wafer. A typical exposure apparatus includes an apparatus frame, an illumination source, a reticle stage, a wafer stage, and an optical assembly that cooperate to transfer an image of an integrated circuit from the reticle onto the semiconductor wafer. The illumination source generates a beam of light energy that passes through the reticle. The optical assembly directs and/or focuses the light passing through the reticle to the wafer.
The sizes of the integrated circuits transferred onto the wafer are extremely small. Accordingly, precise directing and/or focusing of the beam of light energy by the optical assembly is critical to the manufacture of high-density semiconductor wafers.
A typical optical assembly includes a tubular shaped housing and two or more spaced apart optical elements that are secured to the optical housing. Unfortunately, depending upon the wavelength of light energy generated by the illumination source, the type of fluid in the light path, including between the optical elements, can greatly influence the performance of the exposure apparatus. Typically, optical assemblies have air between the optical elements. Air is a gaseous mixture that is approximately twenty-one percent oxygen. Some wavelengths of light energy are absorbed by oxygen. Air also includes water vapor, carbon dioxide and other hydrocarbons, which also absorb significant amounts of the light energy within certain wavelength ranges. Even trace amounts of these unwanted fluids, i.e. ten parts per million or less, can result in significant absorption of the light energy. Absorption of the light energy can lead to losses of intensity and uniformity of the light energy. Moreover, absorption of the light energy can lead to localized heating within the optical assembly. Thus, air within the optical assembly can adversely influence the performance and accuracy of the exposure apparatus. As a consequence, the quality of the integrated circuits formed on the wafer can be adversely influenced.
One solution includes sealing the optical elements to the optical housing to form a sealed optical cavity, and replacing the air in the optical cavity with a weakly absorbing, replacement fluid. The replacement process can include directing the replacement fluid into an inlet of the optical cavity while allowing the air to be pushed from an outlet of the optical cavity. This process is continued until the amount of air in the optical cavity is reduced to an acceptable level.
Unfortunately, the sealed optical cavity can include one or more stagnant flow areas that are not readily purged. Stagnant flow areas experience relatively low or non-existent fluid flow during the normal purging process. These stagnant flow areas are particularly problematic if located in the light path. As a result thereof, a relatively large amount of time and replacement fluid is required to purge the optical cavity.
In light of the above, there is a need for an optical assembly that is purged of any unwanted fluid relatively easily and efficiently. Moreover, there is a need for an optical assembly that minimizes the amount of time required to dilute the unwanted fluid and the amount of replacement fluid used to dilute the unwanted fluid in the optical assembly to acceptable levels. Additionally, there is a need for an exposure apparatus that is capable of generating high-resolution patterns on a semiconductor wafer.